The Associated Press and The New York Times

TEHRAN, Iran — Iran’s reformist-backed presidential candidate surged to a wide lead in early vote counting Saturday, a top official said, suggesting a flurry of late support could have swayed a race that once appeared solidly in the hands of the nation’s ruling clerics.

Former nuclear negotiator Hasan Rowhani’s strong lead may be enough to give him an outright victory and avoid a two-person runoff next Friday.

With long lines at the polls, voting hours were extended by five hours in parts of Tehran and four hours in the rest of the country.

Turnout reached 75 percent, by official count, as disaffected members of the Green Movement, which was crushed in the uprising that followed the disputed 2009 presidential election, dropped a threatened boycott and appeared to coalesce behind Rowhani, and the mayor of Tehran, Mohammad Bagher Qalibaf.

Rowhani had more than 52 percent of the more than 5 million votes tallied, the interior ministry reported, well ahead of Qalibaf, with about 17.3 percent. Hard-line nuclear negotiator Saeed Jalili was third, with about 13 percent. Three other candidates trailed.

It was unclear when the final count would be known. Iran has more than 50 million eligible voters.

The apparently strong turnout suggested liberals and others abandoned a planned boycott as the election was transformed into a showdown across the Islamic Republic’s political divide.

On one side were hard-liners looking to cement their control behind candidates such as Jalili, who says he is “100 percent” against detente with Iran’s foes, or Qalibaf.

Opposing them were reformists and others rallying behind the “purple wave” campaign of Rowhani, the lone relative moderate who’s left in the race.

Even if the last-minute surge for Rowhani brings him to the presidency, it would be more of a limited victory than a deep shake-up. Iran’s establishment — a tight alliance of the ruling clerics and the ultrapowerful Revolutionary Guard — holds all the effective power and sets the agenda on all major decisions, such as Iran’s nuclear program and its dealings with the West.

Security forces also are in firm control after waves of arrests and relentless pressures since the last presidential election in 2009, which unleashed massive protests over claims the outcome was rigged to keep the combative Mahmoud Ahmadinejad in power for a second and final term. He is barred from seeking a third consecutive run.

Rowhani led the influential Supreme National Security Council and was given the highly sensitive nuclear-envoy role in 2003, a year after Iran’s 20-year-old atomic program was revealed. “Rowhani is not an outsider, and any gains by him do not mean the system is weak or that there are serious cracks,” said Rasool Nafisi, an Iranian affairs analyst at Strayer University in Virginia. “The ruling system has made sure that no one on the ballot is going to shake things up.”

Yet a Rowhani victory would make room for more moderate voices in Iranian political dialogue.

It also would bring onto the world stage an Iranian president who has publicly endorsed more outreach rather than bombast toward the West.